Non-Patent Document 1: “Current Techniques of Nanomaterials” written by Mitsue Koizumi et al., first edition, CMC Publishing Co., Ltd., April, 2001
Non-Patent Document 2: “Nanotechnology Handbook, Part I, Creation”, first edition, edited by Nanotechnology Handbook Editorial Committee, Ohmsha, Ltd., May, 2003
Patent Document 1: JP-A H07-144102
Patent Document 2: JP-A 2002-97281
Patent Document 3: JP-A 2007-8924
Patent Document 4: JP-A 2004-49957
Microparticles having a volume-average particle size of 1 μm or less in particle size distribution (referred to hereinafter as nanoparticles), particularly 100 nm or less nanoparticles, have a significantly high ratio of the surface area to volume of the particle, thus bringing about new characteristics, and thus nanomaterials such as nanoparticles themselves, their aggregates or organic-inorganic complexes are highly expected as materials in the 21st century (Non-Patent Document 1). That is, conventional materials can exhibit new functions by mere microparticulation, so the nanoparticles become an important theme in the industrial world, and the advance of nanotechnology is naturally inseparable from microparticles, particularly nanoparticles (Non-Patent Document 2).
Accordingly, a new method for producing microparticles has been desired. Further, it is the most important task in nanotechnology to establish a production method capable of stable large-production in order to utilize the method in industry.
As a method of preparing microparticles in liquid, there is a method of separating microparticles wherein a solution having a specific solute dissolved therein is stirred, while the difference in solubility of the solute in the solvent caused by temperature difference is utilized, as shown in Patent Document 1. This method utilizes the phenomenon that the solubility of the solute in the solvent at a predetermined temperature is decreased by decreasing the temperature of the solvent so that the solute in an amount over saturation solubility is separated. However, since this method uses a general-purpose iron pot having a Faudora vane, preparation of nano size particles is substantially difficult, and when separation of crystals is intended, it is difficult to make their crystal form uniform. Accordingly, a method of obtaining microparticles by dissolving an organic matter in an organic solvent and then pouring the resulting organic matter solution into a solvent in which the organic matter is lower soluble in the organic solvent is becoming popular as shown in Patent Document 2 wherein a good solvent in which a solute is highly soluble is mixed with a poor solvent in which the solute is low soluble, thereby decreasing the apparent solubility of the solute in the good solvent and separating the solute over saturation solubility to obtain microparticles. However, both the two methods described above are those separating a substance in a batch reaction container and a reaction iron pot, wherein stable production of microparticles of nano size or of uniform crystal form is extremely difficult. This is because, in the case of a separating reaction in a batch system generally using stirring operation, temperature gradient or concentration gradient and its accompanying disproportionation of a reaction site inevitably occur. Accordingly, temperature control and concentration control in a batch system are extremely difficult, thus inevitably a uniform reaction difficult. As a result, there arises necessity for prolongation of the reaction time, etc., to make control of all reaction conditions very difficult.
As shown in Patent Document 3, there is a method of obtaining microparticles by utilizing solubility difference with a micromixer or a microreactor. When the general microreactor is used, there are many advantages in the micro-device and system, but as the micro-flow path diameter is decreased, pressure loss is inversely proportional to the biquadrate of the flow path; that is, an extremely high feeding pressure becomes necessary thus making a pump for actually feeding a fluid hardly available. In addition, there are many problems; for example, a phenomena of clogging of a flow path with a product occurs when the reaction is accompanied by separation, a micro-flow path is clogged with bubbles generated by a reaction, a microscopic space is not effective or applicable to every reaction although the speed of molecular diffusion is fundamentally expected for the reaction. Actually, the reaction should be attempted by trial and error in order to select good results. Scaling up has been coped with a method of increasing the number of microreactors, but the number of microreactors which can be stuck is limited to several dozen, thus inherently aiming exclusively at products of high value, and the increase in the number of devices leads to an increase the absolute number of failure causes, and when the problem of clogging actually occurs, it can be very difficult to detect a problem site such as failure site.